I assumed that too, but it doesn't appear to be prohibited.[1] Indeed, I tried 8BSD755 and 9BSD755 which were accepted. (The current sequential plates start with 7.)
> DMV has the right to refuse any combination of letters and/or letters and numbers for any of the following reason(s): [...] it conficts with any regular license plate series issued.
My understanding is that all vanity plates are reviewed by at least one human, so it's possible they handle denying vanity plates that fit the sequential series that probably should have been denied in the online form.
[1] https://www.dmv.ca.gov/wasapp/ipp2/initPers.do